A method for ultrasonic control of the level of liquid in tanks is known (USSR certificate of authorship No. 343155, G 01 F 23/28, 1972), in which a longitudinal ultrasonic wave is introduced into the tank wall at an angle to it and parallel to the surface of the monitored medium; the velocity of the trace of this wave on the input surface is set equal to the velocity of a normal wave (Lamb wave) propagating in the wall, and after passing a fixed distance along the wall, the Lamb wave is received and its amplitude is used as an information signal about the liquid level.
The disadvantage of this method of controlling the liquid level, which reduces its reliability and accuracy, is the instability of the amplitude of the information signal when the thickness of the walls in the tank changes due to corrosion or exposure to aggressive media and overgrowing with residual components of the liquid reagents. The instability of the information signal in this method is also caused by changes in the acoustic resistance of the contact layer between the acoustic transducers and the wall of the tank. These changes can be caused by gradual extrusion of the contact layer due to the temperature fluctuations of the external environment and the associated temperature expansion/contraction.
A device is known (USSR copyright certificate No. 343155, G 01 F 23/28, 1972), which implements this level control method in closed tanks, comprising two identical inclined acoustic transducers directed to each other and mounted on the outer surface of the tank at a fixed distance to each other, serially connected exciter and power amplifier. An amplifier, selector cascade (with its first input), detector and measuring cascade are serially connected to one of the acoustic transducers, the serially connected synchronizer, delay unit, strobe-pulse generator; synchronizer is connected to the input of the exciter, and the strobe-pulse generator to the second input of the first selector cascade, which is connected between the amplifier and the detector.
The disadvantage of this device is low reliability and accuracy caused by the dependence of information signals on the influence of destabilizing factors, for example, the changing acoustic resistance of the contact layer between the acoustic sensors and the tank wall. These changes can be caused by gradual extrusion of the contact layer due to the temperature fluctuations of the external environment and the associated temperature expansion/contraction.
A method for controlling the liquid level is known (RF patent No. 2112221, C1, G 01 F 23/296, 1998), which is that the ultrasonic wave is introduced into the tank wall parallel to the surface of the liquid medium, and the ultrasonic wave propagating in a fixed section of the wall is a Lamb wave; simultaneously using the same source, a longitudinal ultrasonic wave is excited in the tank wall, propagating in the plane of the horizontal section of the tank along the chord of the tank circumference through the wall, the monitored liquid medium and the opposite wall. This wave is received by the same receiver in a time zone different from the Lamb wave reception time zone, the amplitude values of the Lamb wave or the longitudinal wave are extracted in each time zone, which are used to form the normalized information signals at a predetermined sensitivity threshold, which are used to manage the intermediate storage device (ISD), at the output of which one signal is received on the presence of two or at least one of the normalized information signals at the input of the ISD during the period of exciting pulses flow, which are used to judge on the presence of the liquid at a controlled level in the tank.
A disadvantage of the second method for liquid level control is the insufficient reliability due to the fact that the paths and propagation velocities of the longitudinal wave and the Lamb wave are different, so the time interval for their reception can coincide (overlap) with the reception of reflected signals propagated in the tank wall. Besides, the amplitude instability of its signal is due to the non-stable properties of the liquid, the level of which is controlled, for example, by the compressibility β, on which the speed of sound depends. Compressibility, in turn, depends on the temperature, and the nature of this dependence varies from liquid to liquid.
A method for controlling the liquid level is known (RF patent No. 2123172, C1, G 01 F 23/296, 1998). In this method, as well as in the first one, a longitudinal ultrasonic wave is introduced into the wall of the tank at an angle to it, but parallel to the surface of the monitored liquid in the control zone, the velocity of the wave's trace along the input surface is set equal to the velocity of the normal wave (Lamb wave), but, in contrast to the first method, a surface wave is at the same time additionally excited, which does not undergo attenuation when the tank wall is damped with liquid in the level control zone; this wave is received by the same acoustic receiver, the received input signal, which comprises both waves, is weakened. The amplitude of the surface wave is identified in it and detected, and a threshold signal is generated, the value of which is memorized, the amplitude of the surface wave is compared with the threshold signal and when it decreases, a periodic sequence of control pulses is generated, the number of which is proportional to the magnitude of the decrease in the amplitude of the surface wave relative to the threshold signal and inversely proportional to the value of the attenuation, which was initially introduced; the pulses are directed to the attenuation circuits of the input signal, and the amplitude of the information normal wave is restored.
The disadvantages of this method of controlling the liquid level, which reduces its reliability and accuracy, include the fact that it uses a surface wave as a reference signal, velocity of which is less than the Lamb wave velocity, which is used as an information wave. As a result, the time zone for receiving the reference signal can coincide with the action of other waves propagating in the wall of the tank, including reflected ones, which reduce the reliability of its signal identification. The second disadvantage is related to the fact that the initial adjustment and start-up of the liquid level control a priori assumes the presence of a good acoustic contact between the emitter and the receiver, and also the relative invariance of the acoustic characteristics of the paths of the surface wave and the Lamb wave, which can not always be observed.
The device implementing this method (RF patent No. 2123172, G 01 F 23/296, 1998) consists of an acoustic emitter and a receiver, each comprising two piezoelectric transducers mounted at different angles on the wave guide to excite surface and normal Lamb waves in the wall. In this case, the acoustic receiver and the emitter are installed directed to each other on the outer surface of the tank along a line parallel to the surface of the monitored liquid at a fixed distance. Besides, the device comprises an electronic unit of signals generation and extraction, the output of which is connected to the transmitter of acoustic signals, and the input to the receiver of acoustic signals. The electronic conditioning unit includes a synchronizer, the output of which is connected to the exciter, the first input of the coincidence circuit, the first and second delay lines. The output of the exciter is connected to a power amplifier, the output of which is the output of the signal conditioning unit and the input is the input of a summarizer, which output is connected to the first input of the attenuator connected by its output through the amplifier with the second inputs of the two selector channels. The first inputs of these selector channels through the respective conditioners are connected to the outputs of the first and second delay lines. The output of the second selector channel through the second peak detector is connected to the measuring channel. The output of the first selector channel through the first peak detector is connected to the first input of the comparator, the second input of which is connected to the reference voltage master. The output of the comparator is connected to the second input of the coincidence circuit, the output of which, through serially connected counter, decoder and analog key block is connected to the second input of the attenuator.
The disadvantages of this device are low reliability and complexity.
The design of the acoustic emitter and receiver, which comprises two piezoelectric transducers, thus forming two paths of signal conditioning by various elements of the acoustic emitter and receiver, requires the identical influence of external factors, in particular, temperature, on the characteristics of these elements, which is practically difficult and leads to distortion of the final signal informing about the position of the monitored liquid level. Insufficient accuracy is also due to the fact that the hysteresis characteristic of the amplitude of the information signal as a function of the position of the liquid level in the control zone closely adjacent to the line on which the acoustic emitter and receiver are located creates an ambiguous relationship between the set point of the amplitude of the information signal and the position of the liquid level in this zone.
Insufficient reliability of the device is due to its complexity. In its structure, there are elements of the auto-gain control circuit of the received acoustic signals, which itself requires preliminary tuning. Moreover, a priori, it is assumed that such a tuning should be performed with good acoustic contact of the emitter and the receiver with the surface of the tank. Actually, additional complicated technical procedures are required to meet this condition.
A liquid level control device is known (RF patent No. 2383869, G01F 23/28) in which the ultrasonic level meter comprises at least one pair of transducers, emitting and receiving, for exciting and receiving, respectively, of an anti-symmetric Lamb wave in the wall of a container partially filled with liquid, these transducers are mounted on an external surface of the tank, as well as a high-frequency generator, and a synchronization unit connected to the registration unit. In this device, the pairs of transducers are arranged horizontally or vertically relative to the position of the level in the container. The acceptable accuracy of measuring the position of the liquid level relative to the installation position of the transducers is achieved by selecting the optimum frequency (f) of the high-frequency generator associated with the thickness (h) of the tank wall by the relation: f=(3-5)h and the acceptable sensitivity of the receiving equipment, which makes it possible to estimate the change in the amplitude (power) of the gated signals of anti-symmetric Lamb waves.
This device has the same drawbacks as the one described above, in which the amplitude method for measuring the attenuation of Lamb waves is used, the accuracy of which depends, in one way or another, on the initial tuning of the measuring channels of the device and the calibration of its measuring scale. Moreover, this patent considers only cases of either vertical or horizontal arrangement of pairs of transducers and measurements are made only on these individual pairs, and the results of measurements obtained from horizontal pairs are not used to increase the accuracy of measurements obtained from vertical pairs by adjusting (graduating) the measuring scale. Also, the accuracy of the measurement will depend on how correct the oscillator frequency is pre-tuned, which is related with the thickness of the tank wall, the magnitude of which is not always known. It is worth also to be noted that the description of the patent comprises references to the fact that different Lamb waves are excited in the plate (tank wall), but the anti-symmetric Lamb waves of zero order (zero mode) are used by time gating as the most informative. However, the use of an anti-symmetric Lamb wave only (and a symmetric one only, too) is affected by destabilizing factors (for example, temperature, etc.) in the process of measurement, which inevitably makes measurements less accurate.
A method for controlling the liquid level is know (RF patent No. 2437066, G 01 F 23/296, 2010)—selected as a prototype. This method is that a normal wave is periodically introduced by means of an ultrasonic radiator into the tank wall in the control zone, which propagates along the wall of the tank. This wave is received by an acoustic receiver installed at a fixed distance from the emitter, using the energy characteristic of this wave as an information signal for determining the position of the liquid level in the control zone with respect to the installed emitter and receiver, the emitter and receiver being located on the outer surface of the tank in such a manner the acoustic signals from the emitter and receiver are entered at right angles to the surface of the wall of the tank, and they are located throughout the height in such a manner that the liquid level control zone is located between them, exciting and receiving, respectively, simultaneously both symmetric and anti-symmetric Lamb waves of zero mode, then the received signals are digitized by means of an analog-digital conversion, the digital sequences relating to symmetric and anti-symmetric Lamb waves of zero mode are identified, the ratio of the energy characteristics of these waves is calculated, then it is compared with a constant predetermined value and a signal is formed, which indicates the position of the liquid level in the control zone corresponding to a predetermined magnitude of the ratio of signal characteristics.
The disadvantage of this method is the need for preliminary calibration of at least the initial and final values of the measuring scale, which presupposes the fixation of these values with a deliberately filled and drained tank to levels beyond the control zone (above and below the sensor installation area), and knowing and taking into account the parameters of the tank wall, which affect the characteristics of the Lamb wave, which, in general, is not always possible. Without the use of additional level control means that allow the calibration scale to be calibrated directly on site, it is not possible to achieve accurate measurements, as well as high speed of their conduct.
The device implementing the method disclosed in the same patent of the Russian Federation No. 2437066, G 01 F 23/296, 2010, comprises an acoustic emitter and an acoustic receiver including piezoelectric transducers mounted on the outer surface of the tank at a fixed distance from each other, a receiving amplifier, a power amplifier, the output of which is connected to an acoustic emitter, a signal conditioning unit, the input of which is connected to the output of the receiving amplifier, and the output to the input of the power amplifier, and the emitter and receiver of acoustic signals are installed in height in such a manner that the liquid level control zone is located between them, each of which comprises one piezoelectric transducer, the installation of which allows to enter acoustic signals at right angle to the tank wall surface to excite simultaneously symmetric and anti-symmetric normal Lamb waves of zero mode, the amplifier input being connected to the output of the acoustic receiver, and the signal conditioning unit is made in the form of a microcontroller.
This device has the following drawbacks. It requires a preliminary adjustment, consisting in the calibration of at least the initial and final values of the measuring scale, as well as establishing the correspondence of the set point value to the monitored position of the liquid level in the tank. Prior to putting the device into operation, this calibration procedure requires at least one full cycle of draining or filling the tank with liquid in such a way that the liquid level passes through the entire established control zone with the fixation of the required level control points by other means, which is not always possible. For example, in the case of an oil tank with a capacity of several thousand tons, a test drain or fill, even a small level for adjusting the scale, will be impossible for technological reasons due to large volumes of pumping, which can be available only under operating conditions. Another example is a waste storage tank of poisonous waste, in which the limit level is controlled and the discharge from which is made once, after reaching the threshold value of this level. The above drawbacks limit the scope of the device and do not provide accurate measurements within a short period of time (i.e., to perform high-speed fine-tuning to get precise measurements).